Figurative sense of "assault a public reputation" is by 1835; especially "dishonor or besmirch with unsubstantiated charges." Related: Smeared; smearing. Smear-word, one used regardless of its literal meaning but invested with invective, is from 1938.

smear (n.)

"mark or stain left by smearing," 1610s, from smear (v.). Sense of "small quantity prepared for microscopic examination" is from 1903. Meaning "a quantity of cream cheese, etc., smeared on a bagel" is by 1999, from Yiddish shmir. The earliest noun sense in English is "fat, grease, ointment" (c. 1200), from Old English had smeoru "fat, grease," cognate with Middle Dutch smere, Dutch smeer, German Schmer "grease, fat" (Yiddish schmir), Danish smør, Swedish smör "butter."